Measuring flow velocity of fluids is helpful in a variety of applications, including for example research, metering, and monitoring. One approach for monitoring flow is a so-called hot wire anemometer. The hot wire anemometer uses a heated wire positioned within the flow of media, such as gas, liquid, particle-laden liquid, or the like. As the media flows over the hot wire, heat is transferred from the hot wire to the media, cooling the hot wire. Flow rate can be determined from the temperature variation effects on the hot wire.
The hot wire anemometer has two operating modes. The first one is a constant current mode, the flow rate can be measured by a temperature of the hot wire. In this method, the current of the hot wire remains unchanged, when the media takes part of heat away from the hot wire, a temperature of the hot wire will decrease. Therefore, the greater the flow rate, the lower the temperature of the hot wire. When the temperature of the hot wire changes, the resistance of the hot wire will change and a voltage between two ends of the hot wire will change, thus the flow rate can be measured. The second is a constant temperature mode, the flow rate can be measured by a current of the hot wire. In this method, the temperature of the hot wire remains effectively unchanged (the resistance of the hot wire does not change), the current of the hot wire is varied to maintain the original temperature. The greater the flow rate, the greater the current that is needed to maintain the original temperature, thus the flow rate can be measured by the current taken by the hot wire. Calculation principle of the two operating modes of the hot wire anemometer is based on a relationship between temperature and resistance of the hot wire. That is, the greater the effect on resistance that temperature has on the hot wire, the sensitivity of the hot wire anemometer will be correspondingly higher.
According to the law of electrical resistance, the resistance formula of the metal wire is R=ρL/S, where, p is the resistivity of the metal wire, L is the length of the metal wire, S is the cross sectional area of the metal wire. The resistivity ρ is associated with the temperature of the metal wire. When the length of the hot wire is unchanged, the smaller cross sectional area is, the greater the resistance of the hot wire is effected by the temperature. Therefore, the smaller a diameter of the hot wire, the higher will be the sensitivity of the hot wire anemometer.
However, when a diameter of the hot wire made of metal or alloy that is micrometer size or less, the hot wire is easily broken. Therefore, the sensitivity of the conventional hot wire anemometer is bad, and a life of the hot wire is short.